When Mattis announced his departure in December, the White House initially called it a "retirement" and praised the outgoing Cabinet official for "tremendous progress" in strengthening the military. Within a few weeks, however, Trump said called Mattis' performance in the job "not too good" and said his resignation was welcomed.

Now, Trump is suggesting that he fired the popular former Marine Corps general because of poor performance leading the military. In his resignation letter, Mattis wrote that he was stepping down "because you have the right to have a Secretary of Defense whose views are better aligned with yours."

"In the letter he wrote, 'You have to have your own choice,'" Trump said in the interview. "The reason he said that was because I said, 'You're just not my choice.'"

Trump did not elaborate on what changed between his effusive praise of Mattis early in his presidency and the months leading up to his departure. But he did note that "I got him more money than the military has ever seen before" and said he was still unhappy with the results.

In his resignation letter, Mattis took aim at Trump's past criticism of foreign allies and his decision to withdraw U.S. troops from Syria in the months ahead. Mattis had planned a departure date of late February, but Trump announced just three days later that Deputy Defense Secretary Patrick Shanahan would assume the acting defense secretary role on Jan. 1.

Asked whether Shanahan could stay in that role permanently, Trump said the acting secretary is ""doing a terrific job" but also that "a tremendous number of people would like that position."

He also insisted that despite the personnel turnover, "there's really no chaos in the White House."

Trump said one of his top goals for the next two years will be continuing to focus on national security, saying that the administration is still "in the process of rebuilding the military, which was truly depleted."

A Military Times poll conducted in late September found that nearly 84 percent of troops had a favorable view of Mattis' work leading the armed forces as defense secretary. Among officers, the figure was almost 90 percent.

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